In a color picture tube using a shadow mask, the shadow mask, which has lots of fine holes transmitting an electron beam, is disposed in an opposite relation with respect to a phosphor layer formed inside a panel with a predetermined spacing. Surrounding an outer periphery of the shadow mask, a mask frame is fixed. A frame holder is disposed between the mask frame and the panel.
The electron beam that passes through the fine holes of the shadow mask is usually only about 20% of the total electron beam emitted from an electron gun. The remaining about 80% of the electron beam is absorbed by colliding with the shadow mask and the mask frame. When the shadow mask or the mask frame is thermally expanded by collision of the electron beam, the fine holes of the shadow mask and the dots formed in the phosphor layer are shifted in their relative positions to result in degradation of color purity.
Therefore, the shadow mask is generally formed of a low thermal expansion alloy such as an Fe--Ni based alloy and the like. Besides, the mask frame is conventionally formed of a generally used Fe based material from the view point of strength and the like. Therefore, in order to absorb thermal expansion of the mask frame, a spring material which is capable of deforming elastically is employed for a part of the frame holder.
In such a construction, since there is difference in the thermal expansions of the shadow mask and the mask frame when the electron beam is collided, absorption of the thermal expansion solely by the spring material is not enough to result in degradation of the color purity. Thus, a thermal deformation material which deforms in an opposite direction relative to the direction of thermal expansion of the mask frame is used for a part of the frame holder, aside from the spring material. For the thermal deformation member forming a part of such a frame holder, a so-called bimetal obtained by laminating a high thermal expansion member such as an Fe--Ni--Cr based alloy and a low thermal expansion member such as an Fe--Ni based alloy is conventionally used.
In addition, for over current protection, for example, use as a molded-case circuit-breaker and the like, a thermal deformation member is used to break overcurrent by opening a contact point. In the overcurrent protection, the thermal deformation member deforms due to its own Jule heat when the overcurrent flows. Otherwise, the overcurrent flows a resistor (heater) to induce a thermal deformation due to heating of the resistor. A circuit is broken due to deformation of the thermal deformation member. For the thermal deformation member for the over current protection, a 2-layered laminate material obtained by laminating a high thermal expansion member such as an Fe--Ni--Cr based alloy or an Fe--Ni--Mn based alloy and a low thermal expansion member such as an Fe--Ni based alloy, or an Fe--Ni--Co based alloy , or a 3 layered laminate material obtained by interposing Ni or a Cu--Zr alloy and the like as an intermedium is used.
In a bimetal using a high thermal expansion member such as an Fe--Ni--(Cr, Mn) based alloy and a low thermal expansion member such as an Fe--Ni based alloy, according to a required thermal deformation rate, a ratio between plate thicknesses of the high thermal expansion member and the low thermal expansion member is determined. Further, a total thickness of the laminate plate for securing material strength and long term reliability is determined. For example, for a bimetal for a color picture tube and the like, to make possible to secure the material strength and the long term reliability, a total plate thickness of 0.7 mm or more is required.
However, in the conventional bimetal, since the high Ni content alloys such as an Fe--Ni--(Cr,Mn) based alloy and an Fe--Ni based alloy are used for both of the high thermal expansion and low thermal expansion members, there is a problem that manufacturing cost is high. To reduce such a high manufacturing cost of the bimetal, it is considered to reduce concurrently the thicknesses of both of the high thermal expansion member and the low thermal expansion member. However, due to decrease of the total thickness of the bimetal, material strength and long term reliability as a thermal deformation members for both of a color picture tube and an overcurrent protection are deteriorated. Further, both of the Fe--Ni--(Cr,Mn) based alloy and the Fe--Ni based alloy are poor in its workability compared with a generally used Fe based alloy.
In addition, in Japanese Patent Laid-Open Application No. HEI 7-234292, a technology to interpose Ni, Ni alloy, a Zr--Cu alloy as an intermediary layer in order to adjust electric resistance of a bimetal which uses a high thermal expansion member consisting of the Fe--Ni--Cr based alloy and a low thermal expansion member consisting of the Fe--Ni based alloy is disclosed. Further, in Japanese Patent Laid-Open Application No. HEI 3-13889, a clad material which, though its high thermal expansion member is different, is obtained by cladding Cu, Ni, or an alloy formed therebetween as an intermediary member is disclosed. Further, Japanese Patent Laid-Open Application No. SHO 47-13209 discloses a technology in that, because its usage is different from the above described bimetal for a color picture tube, a high thermal expansion member such as a Mo--Cu--Ni based alloy is used for a composite thermostat material. In this patent application, it is disclosed that, in a composite thermostat material having a bimetal structure, various kinds of Fe alloys, which can be manufactured at a low cost compared with the outer side layer, can be used as an intermediary layer in the range that does not make inadequate resistance and flexibility of the composite material.
In sum, a bimetal having an intermediary layer for adjusting electric resistance and a composite thermostat material of bimetal structure having low cost Fe alloy as an intermediary layer were already proposed. However, in a field of the thermal deformation member used for a color picture tube and an overcurrent protection, no satisfactory proposal has been made reduce costs, while satisfying requirement of adequate bent coefficient.
An objective of the present invention is to provide a thermal deformation member for an electron tube for which manufacturing cost is reduced and workability is improved without adversely affecting its strength and long term reliability, and a color picture tube using thereof. Another objective of the present invention is, without adversely affecting its strength and long term reliability, to provide a thermal deformation member for an electric current control for which manufacturing cost is reduced and workability is improved, and an overcurrent protection using thereof.